The Apicomplexa are an eukaryotic protozoan phylum of around 5000 species including parasites which belong to the most successful and devastating pathogens today, infecting a wide range of animals from mollusks to mammals. Many species of Apicomplexa cause diseases of medical and veterinary importance and represent a significant economic burden and global healthcare challenge. Members of the phylum include:                Plasmodium, the etiological agent of malaria, afflicting 10-40% of world population and accounting for one-in-five deaths among children under the age of five in Africa        Toxoplasma gondii, the causative agent of toxoplasmosis. From one-third to half of the world's human population is estimated to carry a Toxoplasma infection. It is a major pathogen to humans with a weakened immune system, such as AIDS patients or pregnant women        Cryptosporidium, a waterborne pathogen which typically does not cause serious illness in healthy people, but is a big health problem for immuno-compromised people, and        the agricultural parasites Eimeria (infects poultry and causes annual losses in revenue totaling nearly a billion dollars), Neospora (an important pathogen in cattle and dogs), Babesia (thought to be the second most common blood parasites of mammals with a major health impact on domestic animals) and Theileria (causative agent of theileriosis a disease of cattle, sheep and goats).        
The apicomplexan life cycle is complex and can be divided into three main stages wherein the first two serve for the asexual replication of the pathogen (more precise of the invasive stages of these protists called sporozoites and merozoites) and the third stage defines the sexual reproduction of the parasite. While the general life cycle is common to the Apicomplexa phylum, there are striking differences between species.
FIG. 1 shows the apicomplexan life cycles. As mentioned above, the members of Apicomplexa share a generalized life cycle, even though each species has its own specializations. Plasmodium spp. and Theileria spp. are transmitted and undergo sexual recombination in an insect vector, the Anopheles mosquito and Rhipicephalus tick, respectively. Cryptosporidium is able to autoinfect its host; the oocyst can sporulate and excyst in the same host, maintaining the infection for months to years. The Coccidian parasites are represented in this figure by Toxoplasma, which is able to infect the majority of warm-blooded animals. The differentiation of Toxoplasma tachyzoites into gametocytes is triggered only when members of the cat family (Felidae) are infected (Wasmuth et al., 2009).
Some Apicomplexa require a single host (e.g. Cryptosporidium), whereas others are more complex, requiring sexual reproduction in the vector species for transmission (e.g. Theileria and Plasmodium; see FIG. 2).
Although members of the Apicomplexa infect different host and cell types, they have a similar number of defining organelles involved in host cell attachment, invasion, and the establishment of an intracellular parasitophorous vacuole within the host cell. The arsenal of organelles varies between species, but typically includes rhoptries, micronemes, and dense granules. To develop novel antiparasitic compounds and increase the understanding of apicomplexan biology, several large-scale-sequencing projects were initiated and the availability of genomic data sets for 15 species opened the way for the identification of conserved protein families and their functions within the phylum and in the above mentioned processes. Domain analysis also identified both the taxonomic distribution of apicomplexan domains as well as domain architectures specific to the Apicomplexa.
Malaria is a disease caused by infection with parasites of the phylum Apicomplexa protozoan, namely parasites of the genus Plasmodium, globally causing more than 200 million new infections and 700 thousand deaths every year. Malaria is especially a serious problem in Africa, where one in every five (20%) childhood deaths is due to the effects of the disease. An African child has on average between 1.6 and 5.4 episodes of malaria fever each year.
Malarial diseases in humans are caused by five species of the Plasmodium parasite: P. falciparum, P. vivax, P. ovale, P. malariae and P. knowlesi, wherein the most prevalent being Plasmodium falciparum and Plasmodium vivax. Malaria caused by Plasmodium falciparum (also called malignantor malaria, falciparum malaria or malaria tropica) is the most dangerous form of malaria, with the highest rates of complications and mortality. Almost all malarial deaths are caused by P. falciparum. 
Briefly, the plasmodial life cycle in man starts with the inoculation of a few sporozoites through the bite of an Anopheles mosquito. Within minutes, sporozoites invade the hepatocyte and start their development, multiplying by schizogony. In the case of P. vivax and P. ovale, some sporozoites may differentiate into hypnozoites, responsible for late relapses of the infection. After a period of 5-14 days—depending on the plasmodial species—schizonts develop into thousands of merozoites that are freed into the bloodstream and invade the red blood cells (RBCs). In the RBC, each merozoite develops into a trophozoite that matures and divides, generating a schizont that, after fully matured, gives rise to up to 32 merozoites within 42-72 h, depending on the plasmodial species. The merozoites, released into the bloodstream, will invade other RBC, maintaining the cycle. Some merozoites, after invading a RBC, develop into sexual forms—the male or female gametocytes which also enter the bloodstream after maturation and erythrocyte rupture. When a female Anopheles mosquito takes its blood meal and ingests the gametocytes, it will become infected. In the mosquito gut, the male gametocyte fuses with the female gametocyte, forming the ookinete, which binds to and passes through the gut wall, remains attached to its external face and transforms into the oocyst. The oocyst will divide by sporogony, giving rise to thousands of sporozoites that are released in the body cavity of the mosquito and eventually migrate to its salivary gland, where they will maturate, becoming capable of starting a new infection in humans when the mosquito bites the host for a blood meal.
Resistance of Plasmodium falciparum to the existing anti-malarial drug chloroquine emerged in the sixties and has been spreading since then. In addition, the malaria parasite has developed resistance to most other anti-malarial drugs over the past decades. This poses a major threat to public health in tropical countries and to travellers. There is every reason to believe that the prevalence and degree of anti-malarial drug resistance will continue to increase. The growing number of insecticide resistant vectors and drug resistant parasites further increases the demand for an effective malaria vaccine. Malaria vaccines are not limited to a single mode of action and hold the potential to dramatically alleviate the burden of malaria.
Some of the difficulties to develop a malaria vaccine result from the multi-stage life cycle of the parasite and its host as mentioned above. Each stage of the parasite development is characterized by different sets of surface antigens, eliciting different types of immune responses. Despite the large variety of displayed surface antigens, the immune response against them is often ineffective. One of the reasons is the extensive sequence polymorphism of plasmodial antigens, which facilitates the immune evasion of the different isolates.
Some of the most prominent blood-stage vaccine candidates MSP1, MSP2, AMA1, and RESA have primarily been selected for clinical testing because of their ability to induce growth-inhibitory antibodies in pre-clinical animal models. However, despite these promising initial data, they have in general proved poorly immunogenic in human volunteers and the induced antibodies were predominantly unable to inhibit the in vitro growth of P. falciparum. 
A pre-erythrocytic vaccine would protect against the infectious form (sporozoite) injected by a mosquito and/or inhibit parasite development in the liver. In a previously unexposed individual, if a few parasites were to escape the immune defences induced by a pre-erythrocytic vaccine, they would eventually enter the blood-stage, multiply within the erythrocytes and establish a full-blown disease.
An erythrocytic or asexual blood-stage vaccine would inhibit the invasion and multiplication of the parasite in the red blood cells, thus preventing (or diminishing) severe disease symptoms during the blood infection. However, it would not prevent the transmission of the parasite.
A sexual-stage vaccine would not protect the person being vaccinated, but instead interrupt the cycle of transmission by inhibiting the development of parasites once they are ingested by the mosquito along with antibodies produced in response to the vaccine. Transmission-blocking vaccines could be part of a multi-faceted strategy directed towards parasite elimination and reduction of overall resistance to anti pre-erythrocytic or erythrocytic treatment.
The before mentioned multi-stage life cycle of malaria parasites presents unique challenges for a synergistic vaccine approach. Immunity against malaria parasites is stage dependent and species dependent. Many malaria researchers and textbook descriptions believe and conclude that a single-antigen vaccine representing only one stage of the life cycle will not be sufficient and that a multi-antigen, multi-stage vaccine that targets different stages of parasite development is necessary to induce effective immunity (Mahajan, Berzofsky et al. 2010). The construction of a multi-antigen vaccine (with the aim of increasing the breadth of the vaccine-induced immune responses to try to circumvent potential P. falciparum escape mutants) can be achieved by either genetically linking (full-size) antigens together, by a mixture of recombinant proteins or by synthetic-peptide-based (15-25-mer), chemically synthesized vaccines containing several peptides derived from different parasite proteins and stages.
A poly-protein approach being comprised of several different antigens or several different alleles of a single antigen (to induce antibodies with synergistic activities against the parasite) is hindered by antigenic diversity and the capacity of P. falciparum for immune evasion (Richards, Beeson, 2009). A large number of antigens have been evaluated as potential vaccine candidates, but most clinical trials have not shown significant impact on preventing clinical malaria although some of them have shown to reduce parasite growth. The size of the resulting fusion protein/vaccine candidate is another limiting factor allowing only the combination of a few selected antigens, not excluding that the chosen antigens are not targets of natural immunity and/or exhibit significant genetic polymorphism. Highly variable antigens with multiple alleles are obviously targets of the immune response under natural challenge, and vaccine studies of AMA1 and MSP2 suggest that allele-specific effects can be achieved (Schwartz, 2012). Practical considerations argue against multi-stage vaccines, particularly the associated increased manufacturing cost of a multicomponent vaccine including several antigens unless these can be encompassed by a single production step and single delivery technology (Hill, 2011). Currently only combination vaccines (being comprised of CSP and AMA1) are undergoing clinical trials that target the pre-erythrocytic and asexual blood stage of P. falciparum (Schwartz, 2012). A multi-antigen vaccine candidate targeting all three life cycle main stages of Plasmodium (including the sexual stage in Anopheles mosquitos and thus blocking parasite transmission) is still not tested in clinical trials.
The so-called SPf66 vaccine was the pioneer multi-epitope, multi-stage peptide-based malaria vaccine. It was first formulated and tested in Colombia (Patarroyo, 1988) and later also manufactured in the USA. SPf66 consists of epitopes of merozoite surface protein 1 (MSP1) linked by a peptide derived from the NANP repeat sequence of the circumsporozoite protein (CSP) adjuvanted with alum, and more recently tested with QS-21 (Schwartz, 2012). Since then, a number of synthetic peptide vaccines have been produced for both murine (P. berghei and P. yoelii) and human (P. falciparum and P. vivax) malarias and tested for immunogenicity or immunogenicity and efficacy. However, in spite of the early momentum, several theoretical considerations and technological hurdles have slowed the progress of this vaccine development approach. A major disadvantage of the peptide-based vaccine approach lies in its limitation to short linear epitopes that lack the surrounding sequence context often required for three-dimensional protein structures (e.g. folded domains) and complex conformational epitopes.
Therefore the availability of novel and improved multi-stage vaccines against parasites of the phylum Apicomplexa would be highly advantageous.